Condensation Inside Bath Exhaust Fan Ducts?!? Help!

9 years ago

Hello. I've searched, but did not see any topic discussing how to fix this issue, or any preventive measures. Thanks in advance for your advice. I cannot figure how millions of homes are built, yet three homes in our neighborhood seem to have this problem.

At least three homes in this small 12 home neighborhood have the same issue. We had the same builder. Our floorplan is very different - the other two homes have very similar floorplans to each other I'll address our issue.

Home is located northern mid-atlantic, just east of the mountain foothills, maybe 1.5 hours west of the coast. Home is custom, built between March and September 2011. We've been moved in just over 3 years.
We opted to upgrade the insulation during construction, and got the new blown insulation (which we LOVE, btw). My husband says it's "blown in rockwood or fiberglass over spray-in closed cell". This is for all above-ground square footage.
Home has a main-level master.

The issue is with at least two bathroom exhaust fans. The fans vent through the roof. I don't think the exhaust fan ducting is insulated - it only goes up from the insulated space, through the colder attic barrier, then through the roof.
So ONLY during cold weather, we get condensation which forms on the INSIDE of the fan duct. This is causing ceiling drywall water damage where the exhaust fan cover meets the drywall, covering the fan blades.
During this last cold spell, temps got down to 5 to 15 degrees for several days. When we ran the bathroom fans, after we turned the fan off, we saw water. From one fan, the water would almost pour - drip very fast & quickly - directly from the fan vent, splashing onto the floor. That one wasn't so bad.
The MBR bath exhaust fan must have a different angle or some aspect of the flashing which allows the exhaust condensation to completely SOAK the surrounding ceiling drywall on that side of the fan. My husband took the fan cover off, and pushed up on the drywall - reminded me of rolled play-dough. There also were blisters of water, which when popped, the water splashed onto the floor tiles. It's bad.

One of our neighbors had his ducts "wrapped" trying to fix. Now we don't yet know WHAT the ducts were wrapped with, but he said this did not work. (our run isn't nearly as long as his - he has very high pitched roof w/no upstairs).

We have confirmed this is NOT the roof leaking. This is not rain coming back in from the flap thing (sorry, I'm not an HVAC lady :-). This is condensation.

So question to you Professionals please, please - How to most other new homes deal with this??? To me, wrapping the ducts with that fat pink insulation (the stuff with a silver skin on one side) would solve - but would it?
The fan seems strong, which I thought was good. Is that an issue??

The builder (punchlist after 3 years, does that give you any clue??) had the idea to rerun and incorporate a bend into the neighbor's duct, forcing a 'trap', and putting some type of tubular water collected there, then let the water run somehow to outside. (sorry, this sounds ludicrous to me).

What is Normal please? Thanks TONS for your ideas or known resolution, or sharing your Standards with us. This is crazy.......

Thanks & Regards - Les.

Comments (3)

cold_weather_is_evil
9 years ago
last modified: 9 years ago
there are two separate issues that need to each be addressed entirely on their own. in all of this, remember that your fan duct is a chimney and reacts accordingly. you usually have warm air flow and waste even when not "using" it.

if you have condensation and it's all in cold weather, insulate the thing. Not with that trendy gimmicky thin plastic junk but with a whole lot of plain old fiberglass. a lot of it. reseal any seams first, perhaps with brush on goop and fiberglass mesh.

the water damage is a separate deal and I would not be any way qualified to suggest anything concrete without eyes-on. both issues do seem to be from poor installs. the s-trap is funny, though.
bpchiil
9 years ago
last modified: 9 years ago
If I am understanding you correctly, you are saying that the duct runs vertically from the fan to the roof? If that is the case, that is one of the biggest causes of your condensation issues.

Duct work should run horizontally for a short run of the duct work, so if condensation does collect (as it is in your case) it cannot find its way back to the interior of the house. Are your existing ducts flexible or metal ducts? If they are metal ducts, I would suggest a different run route as suggested, along with insulating the ducts with wrap. Wrap will work for you, provided you change the run
of the duct work.
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It might be too small for an HVAC person to take it on. It's somewhat hard to recommend because it does involve wiring, and perhaps some siding and flashing work. Many locales require electrical work to be done by a licensed electrician, and you don't want some bozo not doing the flashing correctly if they mess with your siding. You should be able to run the exhaust duct through the joist bays that make up the bathroom ceiling and through the rim of the cantilever. It could exhaust downwards out of the cantilever soffit, but that can sometimes not be the best choice. A small remodeling outfit might be well-versed all aspects of what you need to have done. A capable and conscientious handyman? That can be good or bad, depending on the person. Consider asking for, or use these as points within the conversation: -Rigid duct. Nothing "flex" or with ridges/corrugations. They can use a small section of flex to connect the fan housing to the run of rigid, but there should be no tight radii in the flex or any kinks in the flex. -All seams in the duct sealed with aluminum foil tape. Not "duct tape". -Insulation around the duct, at least within 4' of the rim. -Any horizontal runs in the duct will be pitched down for drainage in case the moist air produces condensate within the duct. -Ask how they will flash the damper into the siding of they run it through the rim. -Ask how they will damp the exhaust if they run it downwards through the cantilever soffit. -Consider getting a timer switch for the fan. You turn the shower light ON, the shower light and exhaust fan turn on. You finish showering and turn the shower light OFF, the light turns off immediately, but the fan continues to run for a time set by you; another 20 or 30 minutes. Do get an idea of how many feet of duct you'll need. Fans are rated to move a certain amount of air, the longer the run of duct and the more turns within that run, the more resistance to flow and the less air the fan can actually move. You might need to up-size the fan to actually move the needed amount of air.
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Ridgid is really the only way to go especially over long runs. We usually spec 4" sched. 20 PVC pipe as it comes in longer lengths than 4" metal does around here , though elbows can be tough to source sometimes. Unfortunately most contractors / HVAC guys will use flexible corrugated hose because it's cheap and fast. If it's a luxury project they'll splurge for the flex aluminum instead of the plastic kind. This type of pipe can cut a fan' effectiveness in half over long runs. Little thought is really given to these fans in most homes today. They are often undersized for a given bathroom and poorly located too. The crappy pipe detailing makes a bad situation worse. Panasonic's website has (or used to) a pretty good tutorial on how to properly size a fan, where to locate it and how to properly pipe it to the outside. I think much of the problem is cost and ignorance. Most people know you get a fan in your bath , but when pressed they won't really be able to tell you its there to expel warm moist air from your home, not smells. They assume that the $34.95 version will be fine for them and a quick & dirty install will get ya out the door for a hundred bucks. The reality is, many master baths, and some hard working kids baths need a much larger fan than 35 bucks can buy. We are often talking $100-$200 for an adequate model, more if you want features or quietness. Add the increased piping cost and labor for the proper venting needed on higher capacity fans and this can quickly become a $500 line item - which starts to look crazy for something as pedestrian looking as a bath fan. Timer switches are also a very good idea as most fans even properly sized ones need to run a while after you've left the bath to completely exhaust the moist air.
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Personally I wouldn't do it. I'd have a external vent for each fan. It's really up to the local code in your area. Most of the time it may be permitted if (and only if) there is a integrated damper on each fan to prevent backdraft. And, some municipal codes require that no two fans could operate at the same time. In the cases where code permits multiple exhaust vents into one duct, then they are the type where there is a central fan (sucking) not several fans (pushing). Plus, there are dampers that must block the bathroom vents that aren't being used. Example link below. You also can't or shouldn't terminate in the attic, inside at the soffit vents or inside into a roof vent. In other words it should be hooked to its own dedicated roof jack/vent. Also the duct work from the fan housing to the roof jack should be insulated to prevent condensation from warm vent air and cold duct. This is commonly overlooked and is a big deal in cold climates. Here's a bit of the code: The IRC is pretty clear on this: (underlining and bold are mine) - M1501.1 Outdoor discharge. The air removed by every mechanical exhaust system shall be discharged to the outdoors. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space. That does not leave any room for any air to be pushed back into the living space by any other mechanical exhaust system. Then the IRC adds this: - M1507.1 General. Where toilet rooms and bathrooms are mechanically ventilated, the ventilation equipment shall be installed in accordance with this section. - M1506.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space or other areas inside the building. That clearly states that exhaust air from the bathroom/toilet room fan shall be exhausted ... directly ... to the outdoors. Based on the above two code sections, unless the installation instructions specifically state that two exhaust fans may be connected to a single duct, then two exhaust fans may not be connected to a single duct. Unless, of course, permitted by local code. Here is a link that might be useful: Ventilator example This post was edited by audiomixer on Sun, Feb 17, 13 at 22:46
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Sierraeast: You can use one outside exhaust vent for both of them. Put a "T" cap with the opposing openings the same size or larger than the opening connecting to the pipe on the outside of the vent tube so that the "T" openings run horizontally. It will prevent back pressure blowing through the vent and possibly mixing toilet air with kitchen air. Any air passing through the horizontal "T" fitting will cause a venturi effect (I'm old enough to remember carburetors.) which will actually suck air out of the vent tube. The lower position of the vent in the bathroom and the position of the range hood relative to the higher placement of the outside exhaust position will form a natural upflow which will be aided by the venturi action of outside air flow across the horizontal "T" cap. This will work well for outside vent placement but will not work if they both vent to an attic crawl space. You need moving air for the "T" to work right Forgive Bama Dude, he's a good guy but undertrained by Trane propaganda. Fairyprincess
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energy_rater_la
9 years ago
last modified: 9 years ago
one has to wonder if cfm of fan
is enough to push humid air
xxx feet UP.

this is why venting through the
side of house or soffit works
so much better.
this is allowed in my climate.
(aside from the fact of why would
you want a hole in the roof for
a bath fan??)

use a smooth vent, as spirals
will slow air flow & trap condensation
unless kept perfectly straight & tight.
pvc, sheet metal work well.
insulation would help, duct wrap
rather than batts. secure the duct
wrap with no gaps or voids, but
don't compress insulation.

best of luck.

Condensation Inside Bath Exhaust Fan Ducts?!? Help!

Comments (3)

cold_weather_is_evil

bpchiil

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energy_rater_la

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